When phase-lock loop (PLL) techniques are used to produce digital filtering techniques, substantially identical circuitry may be used for either high-pass or low-pass filtering action with the inputs and outputs being taken at different places depending upon whether high-pass or low-pass filtering action is desired. Prior art PLL approaches to providing digital high-pass filtering action have not been economically attractive due to the approach contemplated to insure stability. Since the response time of a PLL-type filter becomes excessively long when the input to the controlled oscillator approaches zero, prior art PLL-type filters have only been used with a known range of positive frequency signals.
To the best of the inventor's knowledge, PLL-type digital filters have not been required to deal with positive and negative frequencies as well as the possibility of zero frequency as was required in the application of the present invention. Since there can't be a negative frequency in the analog world, it is doubtful that there is any remotely similar analog PLL-type circuitry.
The present invention was designed in response to a requirement for a filter which would provide a high-pass filter output in response to both positive and negative frequency indication signals as well as being stable in the absence of the occurrence of either of these signals. The output of the present circuit needed to be summed with a digital word signal to slowly increase or decrease the magnitude of that signal to prevent the disruption of signal processing in other circuitry in a telecommunication system.
The present invention accomplishes the high-pass filter function using two counters and a low-pass filter by incorporating a first bias into the divider signal and a complementary bias signal into the up/down counter. Using this approach, the divider signal never approaches zero amplitude and the implementation of the concept uses a minimum number of parts as compared to any known similar concepts in the prior art.
It is thus an object of the present invention to provide an improved high-pass filter.